Ni-based single crystal alloy

ABSTRACT

Disclosed is a single crystal alloy consisting essentially of, by weight, 0.06-0.09% carbon, 0.016-0.035% B, 0.2-0.4% Hf, 0-0.02% Zr, 6.5-8.5% Cr, 0.4-1.0% Mo, 5.5-9.5% W, 1.2-3.1% Re, 8-10% Ta, 0.3-1.0% Nb, 0-0.4% Ti, 4.7-5.4% Al, 0.5-5.0% Co, 0.1-5% Fe, and the balance of Ni and unavoidable impurities. The alloy is free from solidification cracks during casting a large-sized blade of gas turbines, has grain boundary strength sufficient for assuring the reliability during operation, and further has excellent oxidation resistance to a high combustion gas temperature while having excellent high-temperature strength comparable to that of a conventional single crystal alloy.

BACKGROUND OF THE INVENTION

The present invention relates to a novel Ni-based single crystal alloyused for high-temperature parts such as rotor blades and stator bladesin high-temperature apparatuses such as gas turbines, more particularlyto a Ni-based super alloy most suitable for members made of a singlecrystal alloy and used at a high temperature, which large-sized membershave a complicated form and are excellent in high-temperature strengthand high-temperature oxidation resistance.

TECHNICAL BACKGROUND

The combustion gas temperature of gas turbines tend to increase year byyear in order to improve the thermal efficiency thereof, so that amaterial having excellent high-temperature strength is required forcomponents of the gas turbines being exposed to a high temperature.Therefore, the material for a rotor blade, which is exposed to theharshest environment among the components of the gas turbine beingexposed to a high temperature, has changed from an ordinary castingmaterial of a Ni-based supper alloy to a columnar crystal material ofthe Ni-based supper alloy. Further, a single crystal material havingfurther excellent high-temperature strength has been practically used inmany gas turbines for aircraft engines. Also in gas turbines for powergeneration, single crystal blades have been used in some turbine typesbecause an operational temperature of such gas turbines has increasedsignificantly in order to improve the efficiency of the gas turbines.

The columnar crystal material and the single crystal material are onetype of directionally solidified material, and both of them are cast byso called a unidirectional solidification method.

With regard to the columnar crystal material, crystal grains are causedto grow elongationally in a single direction by such a method asdisclosed in U.S. Pat. No. 3,260,505, for example, so as to make grainboundaries perpendicular to an action direction of primary stress aslittle as possible whereby improving the high-temperature strength.

With regard to the single crystal material, an entire casting is made tohave a single crystal by such a method as disclosed in U.S. Pat. No.3,494,709, for example, whereby making it possible to further improvehigh-temperature strength of the material.

Further, in order to improve high-temperature strength of the Ni-basedsupper alloy, solution heat treatment is effective, according to which aprecipitation hardening phase of γ′ is uniformly and finelyprecipitated. Namely, while the Ni-based supper alloy can bestrengthened by precipitation of the γ′ phase primarily consisting ofNi₃(Al, Ti, Nb, Ta), preferably the γ′ phase is precipitated uniformlyand finely.

In the Ni-based supper alloy as solidified, there exists a coarse γ′phase, which consists of a γ′ phase precipitated and coarsened duringcooling after solidified, and of an eutectic γ′ phase coarselycrystallized in a finally solidified region. Thus, it is possible toimprove the Ni-based supper alloy in temperature strength by onceheating it to a high temperature to dissolve the γ′ phase into a matrixγ phase, followed by rapid cooling (i.e. such a heat treatment isreferred to as solution heat treatment), and subsequently subjecting itto aging treatment to cause γ′ phase to uniformly and finelyprecipitate. The solution heat treatment is preferably carried out at atemperature as high as possible that is not lower than a dissolutiontemperature of the γ′ phase and not higher than an initial meltingtemperature.

The reason for this is that as the heat treatment temperature becomeshigh, regions in which the γ′ phase is made uniform and fine increases,and further, as the regions in which the γ′ phase is made uniform andfine increase, the high-temperature strength is improved. Another reasonwhy the high-temperature strength of the single crystal material isexcellent is that a solution heat treatment temperature can be increasedby using a single crystal alloy containing grain boundary strengtheningelements, which considerably decrease a initial melting temperature ofthe alloy, in only a trace amount as small as the impurity level,whereby it is possible to make almost all the γ′ phase coarselyprecipitated after solidification uniform and fine.

As set forth above, the single crystal material of the Ni-based supperalloy is most excellent in high-temperature strength as a material forthe rotor blade of gas turbines in the state of the art. Under such atechnical background, single crystal alloys such as CMSX-4 (refer toU.S. Pat. No. 4,643,782), PWA1484 (refer to U.S. Pat. No. 4,719,080),and Rene'N5 (refer to JP-A-5-59474) have been developed and applied tothe rotor blades of gas turbines in aircraft engines.

However, as stated above, all of those single crystal alloys containgrain boundary strengthening elements such as C, B and Hf in only atrace amount as small as the impurity level. Therefore, if there existcrystal grain boundaries in the rotor blade produced by casting such asingle crystal alloy, the strength of the rotor blade decreasesextremely, and in some cases, longitudinal cracks occur along thecrystal grain boundaries during solidification.

Therefore, in order to use a rotor blade produced by casting the singlecrystal alloy in a gas turbine, it is necessary to make the entirety ofthe rotor blade to have a complete single crystal structure. Since theoverall length of the rotor blade of the gas turbines in aircraftengines is about 100 mm even at maximum, the occurrence probability ofcrystal grain boundaries when casting is low, so that it is possible toproduce the rotor blades made of the single crystal alloy in some degreeof yield.

However, in the case of the rotor blade of the gas turbines for powergeneration, since the overall length of the rotor blade is approximately150 to 450 mm, it is very difficult to make the entirety of the rotorblade to have a complete single crystal structure. Therefore, in thestate of the art, it is difficult to produce the rotor blade of gasturbines for power generation in a high yield with use of the singlecrystal alloy.

On the other hand, the development of an alloy for the columnar crystalmaterial having excellent high temperature strength has been advanced inorder to improve high-temperature strength of a large-sized rotor bladewhich cannot be produced from the single crystal alloy because of itslow casting yield. As a result, columnar crystal alloys such as CM186LC(U.S. Pat. No. 5,069,873) and Rene'142 (U.S. Pat. No. 5,173,255) havebeen developed. These alloys contain grain boundary strengtheningelements in an amount sufficient to ensure reliable operation, and havehigh-temperature strength comparable to that of a first-generationsingle crystal alloy.

However, in the large sized blade, there have been recognized problemsthat solidification cracks are liable to occur during casting, andlongitudinal cracks are liable to occur along crystal grain boundariesduring solidification due to increased thermal stress caused by the risein combustion gas temperature.

With regard to such problems, for the purpose of obtaining an alloycomposition compatibly having high-temperature strength and grainboundary strength, a JP patent application has been filed (refer toJP-A-9-272933) according to which four types of grain boundarystrengthening elements of C, B, Hf and Zr are added to a single crystalalloy in various combinations of the additive elements whereby anexamination has been conducted with regard to relationships amongadditive amounts of the grain boundary strengthening elements,high-temperature strength, grain boundary strength and solution heattreatment.

Further, a JP patent application (JP-A-2002-146460) has been filedaccording to which 0.1% Si is added to a single crystal alloy in orderto improve oxidation resistance property thereof.

However, since this alloy does not contain C, B and Hf, and is subjectedto complete solution heat treatment, it is unnecessary to considerdeterioration of creep strength due to additive Si, whereas in the caseof such an alloy shown in JP-A-9-272933 which contains C, B and Hf andis subjected to partial solution heat treatment, there is a problem thatSi can not be simply added to the alloy.

Further, there has been proposed a method of remarkably improving theoxidation resistance property of a single crystal alloy, according towhich several tens ppm of a rare earth element is added to the singlecrystal alloy (refer to JP-A-2004-197216).

However, since rare earth elements are active and they reacts with amold or a core during casting the single crystal alloy, there areproblems that it is difficult to control residual amounts of the rareearth elements in the alloy by the reasons that since rare earthelements are active and they reacts with a mold or a core during castingthe single crystal alloy, hetero-crystals are liable to occur wherebynot only making it very difficult to cast a large-sized rotor-blade madeof a single crystal alloy but also it is difficult to control residualamounts of the rare earth elements in the single crystal alloy becausethe additive amounts of the elements are fully consumed by the reaction.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a Ni-based singlecrystal alloy which is free from solidification cracks during casting alarge-sized blade of gas turbines, has grain boundary strengthsufficient for assuring the reliability during operation, and furtherhas excellent oxidation resistance to a high combustion gas temperaturewhile having excellent high-temperature strength comparable to that of aconventional single crystal alloy.

A key aspect of the invention Ni-based single crystal alloy resides inthat the invention alloy is based on a single crystal alloy containingthree types of grain boundary strengthening elements of C, B and Hf, towhich single crystal alloy Fe and Si are added in place of reactive rareearth elements for the purpose of obtaining excellent properties ofhigh-temperature strength, grain boundary strength and oxidationresistance which have been considered to be inconsistent with oneanother according to the prior art, whereby significantly improvingcorrosion resistance property while maintaining mechanical strength in asingle crystal state, or in a state having crystal grain boundaries.

According to a first feature of the present invention, the Ni-basedsingle crystal alloy has excellent properties of high-temperaturestrength, grain boundary strength, and oxidation resistance, andconsists essentially of, by weight, from not less than 0.06% to not morethan 0.09% carbon, from not less than 0.016% to not more than 0.035% B,from not less than 0.2% to not more than 0.4% Hf, from inclusive zero tonot more than 0.02% Zr, from not less than 6.5% to not more than 8.5%Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than5.5% to not more than 9.5% W, from not less than 1.2% to not more than3.1% Re, from not less than 8% to not more than 10% Ta, from not lessthan 0.3% to not more than 1.0% Nb, from inclusive zero to not more than0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not lessthan 0.5% to not more than 5.0% Co, from not less than 0.1% to not morethan 5% Fe, and the balance of Ni and unavoidable impurities.

According to a second feature of the present invention, the Ni-basedsingle crystal alloy has excellent properties of high-temperaturestrength, grain boundary strength, and consists essentially of, byweight, from not less than 0.06% to not more than 0.08% carbon, from notless than 0.016% to not more than 0.035% B, from not less than 0.2% tonot more than 0.3% Hf, from inclusive zero to not more than 0.02% Zr,from not less than 6.9% to not more than 7.3% Cr, from not less than0.7% to not more than 1.0% Mo, from not less than 7.0% to not more than9.0% W, from not less than 1.2% to not more than 1.6% Re, from not lessthan 8.5% to not more than 9.5% Ta, from not less than 0.6% to not morethan 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from notless than 4.9% to not more than 5.2% Al, from not less than 0.8% to notmore than 1.2% Co, from not less than 0.1% to not more than 5% Fe, andthe balance of Ni and unavoidable impurities.

According to a third feature of the present invention, the Ni-basedsingle crystal alloy has excellent properties of high-temperaturestrength, grain boundary strength, and oxidation resistance, andconsists essentially of, by weight, from not less than 0.06% to not morethan 0.09% carbon, from not less than 0.016% to not more than 0.035% B,from not less than 0.2% to not more than 0.4% Hf, from inclusive zero tonot more than 0.02% Zr, from not less than 6.5% to not more than 8.5%Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than5.5% to not more than 9.5% W, from not less than 1.2% to not more than3.1% Re, from not less than 8% to not more than 10% Ta, from not lessthan 0.3% to not more than 1.0% Nb, from inclusive zero to not more than0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not lessthan 0.5% to not more than 5.0% Co, from not less than 0.5% to not morethan 3% Fe, and the balance of Ni and unavoidable impurities.

According to a fourth feature of the present invention, the Ni-basedsingle crystal alloy has excellent properties of high-temperaturestrength, grain boundary strength, and oxidation resistance, andconsists essentially of, by weight, from not less than 0.06% to not morethan 0.08% carbon, from not less than 0.016% to not more than 0.035% B,from not less than 0.2% to not more than 0.3% Hf, from inclusive zero tonot more than 0.02% Zr, from not less than 6.9% to not more than 7.3%Cr, from not less than 0.7% to not more than 1.0% Mo, from not less than7.0% to not more than 9.0% W, from not less than 1.2% to not more than1.6% Re, from not less than 8.5% to not more than 9.5% Ta, from not lessthan 0.6% to not more than 1.0% Nb, from inclusive zero to not more than0.4% Ti, from not less than 4.9% to not more than 5.2% Al, from not lessthan 0.8% to not more than 1.2% Co, from not less than 0.5% to not morethan 3% Fe, and the balance of Ni and unavoidable impurities.

According to a fifth feature of the present invention, the Ni-basedsingle crystal alloy has excellent properties of high-temperaturestrength, grain boundary strength, and oxidation resistance, andconsists essentially of, by weight, from not less than 0.06% to not morethan 0.09% carbon, from not less than 0.016% to not more than 0.035% B,from not less than 0.2% to not more than 0.4% Hf, from inclusive zero tonot more than 0.02% Zr, from not less than 6.5% to not more than 8.5%Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than5.5% to not more than 9.5% W, from not less than 1.2% to not more than3.1% Re, from not less than 8% to not more than 10% Ta, from not lessthan 0.3% to not more than 1.0% Nb, from inclusive zero to not more than0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not lessthan 0.5% to not more than 5.0% Co, from not less than 1% to not morethan 3% Fe, and the balance of Ni and unavoidable impurities.

According to a sixth feature of the present invention, the Ni-basedsingle crystal alloy having excellent properties of high-temperaturestrength, grain boundary strength, and oxidation resistance inaccordance with the present invention consists essentially of, byweight, from not less than 0.06% to not more than 0.09% carbon, from notless than 0.016% to not more than 0.035% B, from not less than 0.2% tonot more than 0.4% Hf, from inclusive zero to not more than 0.02% Zr,from not less than 6.5% to not more than 8.5% Cr, from not less than0.4% to not more than 1.0% Mo, from not less than 5.5% to not more than9.5% W, from not less than 1.2% to not more than 3.1% Re, from not lessthan 8% to not more than 10% Ta, from not less than 0.3% to not morethan 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from notless than 4.7% to not more than 5.4% Al, from not less than 0.5% to notmore than 5.0% Co, from not less than 0.1% to not more than 5% Fe, fromnot less than 0.01% to not more than 0.2% Si, and the balance of Ni andunavoidable impurities.

According to a seventh feature of the present invention, the Ni-basedsingle crystal alloy has excellent properties of high-temperaturestrength, grain boundary strength, and oxidation resistance, andconsists essentially of, by weight, from not less than 0.06% to not morethan 0.09% carbon, from not less than 0.016% to not more than 0.035% B,from not less than 0.2% to not more than 0.4% Hf, from inclusive zero tonot more than 0.02% Zr, from not less than 6.5% to not more than 8.5%Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than5.5% to not more than 9.5% W, from not less than 1.2% to not more than3.1% Re, from not less than 8% to not more than 10% Ta, from not lessthan 0.3% to not more than 1.0% Nb, from inclusive zero to not more than0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not lessthan 0.5% to not more than 5.0% Co, from not less than 0.1% to not morethan 5% Fe, from not less than 0.05% to not more than 0.15% Si, and thebalance of Ni and unavoidable impurities.

According to another aspect of the present invention, there is provideda casting made of a Ni-based single crystal alloy having excellentproperties of high-temperature strength, grain boundary strength, andoxidation resistance, wherein the Ni-based single crystal alloy consistsessentially of, by weight, from not less than 0.06% to not more than0.09% carbon, from not less than 0.016% to not more than 0.035% B, fromnot less than 0.2% to not more than 0.4% Hf, from inclusive zero to notmore than 0.02% Zr, from not less than 6.5% to not more than 8.5% Cr,from not less than 0.4% to not more than 1.0% Mo, from not less than5.5% to not more than 9.5% W, from not less than 1.2% to not more than3.1% Re, from not less than 8% to not more than 10% Ta, from not lessthan 0.3% to not more than 1.0% Nb, from inclusive zero to not more than0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not lessthan 0.5% to not more than 5.0% Co, from not less than 0.1% to not morethan 5% Fe, and the balance of Ni and unavoidable impurities.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, there will be provided an explanation of reasons for settingthe component ranges and further preferred other limitation requirementsof the invention Ni-based single crystal alloy having excellentproperties of high-temperature strength, grain boundary strength, andoxidation resistance.

B: 0.016 to 0.035%

Boron is an element for securing both of strength along thesolidification direction and strength along the direction perpendicularto the solidification direction, that is, both of the high-temperaturestrength and the grain boundary strength, but has a characteristics ofconsiderably decreasing the initial melting temperature of alloy. In thecase where a much amount of boron is added into the Ni-based singlecrystal alloy, it will be needed to consider an influence to lowering ofthe initial melting temperature of the alloy. In the invention alloy,the boron amount most suitable for both of the strength along thesolidification direction and the strength along the directionperpendicular to the solidification direction is in the range from morethan 0.015% to not more than 0.04%. Particularly, in the vicinity of0.025% boron, both of the strength along the solidification directionand the strength along the direction perpendicular to the solidificationdirection are maximum.

C: 0.06 to 0.09%

Carbon is an important element for making both of the high-temperaturestrength and the grain boundary compatible with each other. Creeprupture strength along the solidification direction decreases as thecarbon amount increases. In contrast, the creep rupture strength alongthe direction perpendicular to the solidification direction at thecrystal grain boundary is improved as the carbon amount increases in therange of not more than 0.20%, preferably not more than 0.10%. Therefore,it is believed that the mount range of carbon most suitable for securingboth of the high-temperature strength and the grain boundary strength isfrom 0.06% to less than 0.09%. If the carbon amount is not more than0.05%, although the high-temperature strength is excellent, the grainboundary strength is low, so that it is impossible to preventsolidification cracks during casting and ensure reliable operation. Onthe other hand, if the carbon amount is 0.1% or more, thehigh-temperature strength decreases considerably. Therefore, the carbonamount should be in the range of 0.06 to 0.09%, preferably 0.06 to0.08%.

Zr: not more than 0.02%

Zirconium considerably lowers the initial melting temperature of alloywhereby making the solution heat treatment at a high temperatureimpossible resulting in that creep rupture strength along thesolidification direction of the alloy is deteriorated. This element doesnot contribute to improvement of the creep rupture strength along thetransverse direction and the oxidation resistance property. For thesereasons, preferably the Zr amount is less than 0.02%, and morepreferably Zr should not be substantially added.

Hf: 0.2 to 0.3%

Like as Zr, hafnium considerably lowers the initial melting temperatureof alloy whereby making the solution heat treatment at a hightemperature impossible resulting in that the creep rupture strengthalong the solidification direction and the creep rupture strength alongthe transverse direction of alloy are deteriorated.

However, Hf improves tensile ductility along the transverse direction.Further, the Hf amount of about 0.25% improves both of the creep rupturestrength and the tensile strength along the transverse directionalthough somewhat decreasing the creep rupture strength along thesolidification direction. Thus, the optimum Hf amount is from 0.2% to0.4%.

Ta: 8.0 to 10.0%

Tantalum is desirably added in an amount of not less than 8.0% in orderto improve the high-temperature strength. On the other hand, a lot ofadditive Ta increases the solutioning temperature of the γ′ phase. Thus,if Ta is added excessively, a difference between the initial meltingtemperature of alloy and the solutioning temperature of γ′ phase becomessmall, so that a temperature zone in which the γ′ phase can besolutioned without occurrence of initial melting decreases, and anamount of precipitates for precipitation strengthening of the alloydecreases. Thus, the additive amount of Ta exceeding 12% has no effectof improving high-temperature strength of the alloy, so that the upperlimit of the additive amount of Ta is preferably not more than 10%, andthe optimum value of the additive amount of Ta for improvinghigh-temperature strength of the alloy is in the range of 8.5 to 9.5%.

Co: 0.5 to 5%

Cobalt decreases high-temperature strength of the alloy as the Co amountincreases. Therefore, considering the high-temperature strength, the Coamount should be not more than 5%, preferably in the range of 0.5% to1.2%. An additive Co amount of 0.8% to 1.2% is effective in improvementof corrosion resistance property without decreasing high-temperaturestrength of the alloy.

W: 5.5 to 9.5%

Tungsten is effective in improvement of high-temperature strength of thealloy by solid solution strengthening. A desirable additive amount oftungsten is not less than 5.5%. In the case where importance is attachedto high-temperature strength of the alloy, preferably the additivetungsten amount is not less than 7.0%. On the other hand, the effect ofadditive tungsten saturates at a certain additive amount, and anexcessive amount tungsten deteriorates high-temperature strength of thealloy. This is because if tungsten is added excessively exceeding asoluble limit, acicular or tabular precipitates consisting primarily oftungsten occur. Therefore, the upper limit of additive tungsten shouldbe 9.5%, preferably 9.0%.

Re: 1.2 to 3.1%

Like as tungsten, rhenium is effective for improvement of the alloy inhigh-temperature strength by solid solution strengthening. A desirableadditive amount of Re is not less than 1.2%. On the other hand, theeffect of Re saturates at a certain additive amount, and if it is addedexcessively, the alloy will be deteriorated in high-temperaturestrength. This is because if Re is added excessively exceeding thesoluble limit, acicular or tabular precipitates consisting of Re occur.Therefore, the upper limit of additive Re should be 3.1%, preferably1.6%.

Since W and Re exhibit almost the same behavior in the alloy, theoptimum additive amount of those is preferably thought as those totalamount of W and Re. High-temperature strength of the alloy is maximum inthe range of 9.5 to 12% of “W+Re” in total. In contrast, if the “W+Re”amount is less than 9.5%, the high-temperature strength will bedeteriorated because the solid solution strengthening effect isinsufficient. Also, if the “W+Re” amount exceeds 12%, a lot of theaforementioned precipitates will occur resulting in that creep strengthof the alloy is considerably deteriorated at a temperature of not lowerthan 1000° C.

Al: 4.7 to 5.4%

Aluminum is an indispensable element in order to form the γ′ phase whichis a factor of strengthening the Ni-based supper alloy. Also, aluminumcontributes to improvement of oxidation and corrosion resistanceproperties by forming an Al₂O₃ film on the surface of the alloy.Therefore, the Al amount is preferably not less than 4.7% at theminimum. However, if Al is added excessively exceeding 6.5%, a quantityof eutectic γ′ phase n the alloy increases. The invention alloy has beenso designed that that it can exhibit excellent high-temperature strengthby optimizing the additive amount of elements effective for solidsolution strengthening, even if the alloy is in the state of incompletesolution heat treatment. Thus, even if the eutectic γ′ phase exists inthe alloy, it has excellent high-temperature strength. However, in thecase of creep damage, the eutectic γ′ phase finally becomes a startpoint of occurrence of crack to put forward a breakage time of the alloymaterial, so that preferably a quantity of the eutectic γ′ phase issmall. Therefore, the additive Al amount is preferably made not morethan 5.4%, more preferably in the range of 4.9 to 5.2%.

Cr: 6.5 to 8.5%

Chromium forms a Cr₂O₃ film on the surface of the alloy to improve thealloy in corrosion and oxidation resistance properties. Thus, desirablythe Cr amount is 6.5% at the minimum. However, if Cr is addedexcessively, the formation of the aforementioned precipitates of W andRe will be promoted, so that there arises a need for decreasing theadditive amount of W or Re effective for ensuring high-temperaturestrength of the alloy. Thus, in the case where importance is attached tothe high-temperature strength, preferably the Cr amount is not more than8.5%, more preferably in the range of 6.9 to 7.3%.

Mo: 0.4 to 1.0%

Molybdenum exhibits the same effect as that of W and Re, butconsiderably deteriorates oxidation resistance property of the alloy ina high-temperature atmosphere. Thus, in the case where importance isattached to the oxidation resistance property, it is desirable torestrict the Mo amount to not more than 1%. In the case where the alloyrequires the corrosion resistance property in some degree, the additiveMo amount is preferably 0.7 to 1%.

Nb: 0.3 to 1.0%

Niobium is one element belonging to a group including Ta, and has almostthe same effect as that of Ta in improvement of high-temperaturestrength of the alloy. 0.3 to 4% Nb can be contained in the alloy. SinceNb is liable to form sulfides in an environment in which a fuelcontaining a much amount of sulfur is used, it has an effect of delayingsulfur invasion into the alloy whereby improving corrosion resistanceproperty of the alloy. In the present invention, however, it has beenrevealed that, in the case where a certain or larger amount of Nb and Bexists in the alloy, a low melting point phase consisting primarily ofNb and B is formed in an eutectic region whereby considerably loweringthe initial melting temperature of the alloy. The low melting pointphase is formed by segregation. Depending on a casting condition, thelow melting point phase may be or may not be formed. However, in thecase where the low melting point phase is formed, the solution heattreatment at a high temperature cannot be carried out, so thathigh-temperature strength of the alloy cannot be improved. Also, in thecase where a result of the preliminary study about solution heattreatment on a specimen alloy cast under a condition, in which the lowmelting point phase is not formed, is applied to another specimen alloywhich has the same composition as the above specimen but is cast under acondition in which the low melting point phase is formed, a low meltingpoint region of the alloy melts partially whereby high-temperaturestrength of the alloy is considerably deteriorated. From the above, inthe present invention, the additive Nb amount is preferably 0.3 to 1%,more preferably 0.6 to 1.0%.

Ti: not more than 0.4%

Like as Nb, titanium is liable to form sulfides, and has an effect ofimproving the alloy in corrosion resistance property in an environmentin which a fuel containing a much amount of sulfur is used. However,since Ti lowers the melting point of an eutectic region like as Nb, inthe present invention, the additive Ti amount is set to be not more than0.4%. Ti should not be intentionally added except for the case ofimpurities. If an amount of Ti contained as an impurity is not more than0.2%, the alloy is not affected in alloy properties. Therefore,preferably the Ti amount is not more than 0.2%.

Fe: 0.1 to 5.0%

Iron is an element with which Ni is easily replaceable, and has beenbelieved to be an element which deteriorates creep strength of theNi-based alloy. Also, since oxidation resistance property of Fe itselfis poor, Fe contained in the Ni-based alloy deteriorates the alloy inoxidation resistance property. Thus, in conventional single crystalalloys, Fe has been regarded as an impurity, so that in general the Feamount has been limited to be not more than 0.02%.

In the present invention, a new effect of Fe was first discovered. Thepresent invention has overturned common sense, which reveals thatseveral percent of additive iron does not deteriorate creep strength ofthe Ni-based alloy, and on the contrary improves oxidation resistanceproperty of the alloy at a high temperature.

With regard to high-temperature strength of the alloy in the case whereNi is replaced with Fe, if an excessive amount of more than 5% Fe isadded in the alloy, high-temperature strength of the alloy isdeteriorated, so that the Fe amount is preferably limited to not morethan 5%. On the other hand, Fe improves oxidation resistance property ofthe alloy when the Fe amount is not less than 0.1%. Thus, in the casewhere importance is attached to the oxidation resistance at a hightemperature, not less than 0.1% Fe is preferably added in the alloy. Inorder to obtain both the effects in the present invention, preferablythe Fe amount is 0.1 to 5%, more preferably 0.5 to 3%, and still morepreferably 1 to 3%.

Si: 0.01 to 0.2%

Silicon is a replaceable element with Al, and enters into the γ′ phaseof the Ni-based alloy. Si in the γ′ phase changes the lattice constantof the γ′ phase, and deteriorates creep strength. On the other hand, ithas been known that Si improves oxidation resistance property of thealloy. In conventional single crystal alloys, since importance has beenattached to the creep strength, Si has been regarded as an impurityelement, so that in general the Si amount has been limited to not morethan 0.01%.

The present invention is characterized by a combined effect by means ofFe and Si. The present inventors found a new effect that when Si isadded in an alloy containing several percent of Fe, oxidation resistanceproperty of the alloy is improved without deterioration of creepstrength of the alloy. The effect of Si of improving the oxidationresistance property is obtained when the Si amount is not less than0.01%. Thus, in the case where importance is attached to the oxidationresistance property at a high temperature, not less than 0.01% Si ispreferably added in the alloy. In order to prevent the creep strengthfrom deterioration, the upper limit of the Si amount is preferably 0.2%.In order to achieve both the effects in the present invention, the Siamount is preferably 0.01 to 0.2%, more preferably 0.05 to 0.15%.

ADVANTAGES OF THE INVENTION

The present invention relates to the Ni-based single crystal alloy whichis free from solidification cracks during casting a large-sized blade ofgas turbines, has grain boundary strength sufficient for ensuring thereliability during operation, and has both of excellent properties ofhigh-temperature strength and oxidation resistance. With utilization ofcomponents made of the invention alloy in gas turbines, which componentsare exposed to a high temperature, advantageously it is possible toraise combustion temperature of the gas turbines, and improve a powergeneration efficiency of gas turbines for power generation.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship between creep rupture timeand Fe amount in the present invention;

FIG. 2 is a diagram showing the relationship between weight change andFe amount in an oxidation test in the present invention;

FIG. 3 is a diagram showing the relationship between creep rupture timeand Si amount in the present invention; and

FIG. 4 is a diagram showing the relationship between weight change andSi amount in an oxidation test in the present invention.

EXAMPLE

On the basis of a single crystal alloy containing three grain boundarystrengthening elements of C, B and Hf, a Ni-based single crystal alloywas achieved by adding Fe and Si into the base single crystal alloy. Thethus achieved Ni-based single crystal alloy is free from solidificationcracks during casting a large-sized blade of gas turbines, has grainboundary strength sufficient for ensuring the reliability duringoperation, and has excellent oxidation resistance to a high combustiongas temperature while having excellent high-temperature strengthcomparable to that of conventional single crystal alloys.

Table 1 shows chemical compositions of the present invention alloys,which were produced by vacuum induction melting with utilization of abase alloy disclosed in JP-A-9-272933. Particularly a master ingot wasfirst produced. Next, a single crystal specimen, having a diameter of 15mm and a length of 180 mm, was cast with use of the master ingot in aunidirectional solidification furnace. The casting temperature was setat 1540° C., and the solidification rate was set at 20 cm/h. Aftercasting, the specimens were subjected to multi-stage solution heattreatment, according to which the heat treatment temperature waselevated from 1250° C. to 1280° C. incrementally by 10° C., wherein thespecimens were held for 4 hours at each temperature stage from 1250° C.to 1280° C. After the multi-stage solution heat treatment, the specimenswere cooled in air. After the solution heat treatment, the specimenswere subjected to aging heat treatment was such that the specimens wereheld at 1080° C. for 4 hours followed by air cooling at first, and nextthey were held at 871° C. for 20 hours followed by air cooling.Thereafter, the specimens were machined, and subjected to a creeprupture test and an oxidation test.

TABLE 1 (wt %) Specimen No. Ni C B Co Cr Mo W Ta Re Al Hf Nb Ti Zr Fe SiBase Bal. 0.07 0.020 1.0 7.2 0.9 8.8 8.8 1.4 5.0 0.3 0.8 <0.1 <0.02<0.02 <0.01 alloy Y-10A Bal. 0.07 0.017 1.0 7.0 0.8 8.8 8.8 1.4 5.1 0.30.8 <0.1 <0.02 0.5 <0.01 Y-10B Bal. 0.07 0.018 0.9 7.1 0.8 8.9 8.8 1.45.1 0.3 0.8 <0.1 <0.02 1.0 <0.01 Y-10C Bal. 0.07 0.020 1.0 7.2 0.8 8.88.9 1.5 5.2 0.2 0.7 <0.1 <0.02 2.0 <0.01 Y-10D Bal. 0.07 0.017 1.0 7.20.8 8.7 8.9 1.4 5.2 0.2 0.8 <0.1 <0.02 3.0 <0.01 Y-10E Bal. 0.06 0.0161.0 7.2 0.8 8.8 8.8 1.4 5.1 0.2 0.7 <0.1 <0.02 5.0 <0.01 Y-20A Bal. 0.060.020 1.1 7.2 0.9 8.9 8.9 1.4 5.1 0.2 0.7 <0.1 <0.02 <0.02 0.05 Y-20BBal. 0.06 0.020 1.0 7.1 0.7 8.8 8.9 1.3 5.2 0.2 0.7 <0.1 <0.02 <0.020.10 Y-20C Bal. 0.07 0.017 1.0 7.2 0.8 8.9 8.8 1.5 5.2 0.3 0.7 <0.1<0.02 <0.02 0.20 Y-20D Bal. 0.06 0.020 1.0 7.2 0.8 8.7 8.9 1.4 5.2 0.20.8 <0.1 <0.02 <0.02 0.50 Y-30A Bal. 0.07 0.018 1.0 7.1 0.9 8.9 8.9 1.45.1 0.2 0.8 <0.1 <0.02 1.0 0.05 Y-30B Bal. 0.07 0.018 1.0 7.2 0.7 8.88.8 1.3 5.1 0.3 0.9 <0.1 <0.02 1.0 0.10 Y-30C Bal. 0.07 0.019 0.9 7.10.8 8.7 8.9 1.5 5.2 0.2 0.7 <0.1 <0.02 2.0 0.10 Y-30D Bal. 0.07 0.0241.0 7.1 0.7 8.9 8.9 1.5 5.2 0.3 0.7 <0.1 <0.02 2.5 0.15

Table 2 shows the results of the creep rupture test and the oxidationtest. The creep rupture test was carried out under stress of 14 kg/mm²at a temperature of 1040° C. The oxidation test was carried out byholding the specimens at 1150° C. for 100 hours repeatedly until a totaltime of 1,000 hours.

TABLE 2 Creep test Oxidation test Specimen Fe Si Rupture time (h) Weightchange (mg) No. (wt %) (wt %) (1040° C.-137 MPa) (1100° C./1000 h) Base<0.02 <0.01 643.7 −52.6 alloy — −57.6 Y-10A 0.5 <0.01 643.4 −38.1 Y-10B1.0 <0.01 513.4 −29.9 Y-10C 2.0 <0.01 641.9 −32.5 Y-10D 3.0 <0.01 599.5−31.2 Y-10E 5.0 <0.01 370.3 −47.1 Y-20A <0.02 0.05 601.4 −35.5 Y-20B<0.02 0.10 411.3 −21.2 Y-20C <0.02 0.20 333.5 −23.3 Y-20D <0.02 0.50206.3 −19.9 Y-30A 1.0 0.05 594.8 −22.4 Y-30B 1.0 0.10 520.4 −20.1 Y-30C2.0 0.10 711.0 −18.4 Y-30D 2.5 0.15 612.7 −15.7

FIG. 1 shows the relationship between creep rupture time and the Feamount. In the case where the Si amount is not more than 0.01%, thecreep strength scarcely decreases when the Fe amount is not more than3%, and decreases when the Fe amount is 5%.

On the other hand, for the alloy containing 0.05 to 0.15% Si, the creepstrength becomes maximum when the Fe amount is about 2%. Seeing FIG. 2which shows the relationship between the weight change and the Fe amountin the oxidation test, it can be understood that as the Fe amountincreases, the weight change (a weight decrease due to oxidation)decreases, and the oxidation resistance is improved.

FIG. 3 shows the relationship between the creep rupture time and the Siamount. From FIG. 3, it can be seen that in the case where the Fe amountis not more than 0.02%, as the Si amount increases, the creep strengthdecreases.

Seeing FIG. 4 which shows the relationship between the weight change andthe Si amount in the oxidation test, it can be understood that as the Siamount increases, the weight change (a weight decrease due to oxidation)decreases, and the oxidation resistance is improved.

Seeing FIGS. 3 and 4, it can be understood that in the case where the Feamount is not more than 0.02%, both of the creep strength and oxidationresistance are compatible up to 0.1% Si, and for the alloy containing1.0 to 2.5% Fe, both of the creep strength and the oxidation resistanceare compatible up to about 0.2% Si.

The master ingots of the base alloy and Y-10C shown in Table 1 wereproduced by vacuum induction melting, and subsequently unidirectionallysolidified flat plates, each having a size of 15 mm×100 mm×220 mm, werecast in a furnace for unidirectional solidification. These alloys weresubjected to solution heat treatment and aging heat treatment under thesame conditions as those of the first example, thereafter a creeprupture test was performed under the conditions of a temperature of 927°C. and stress of 32 kg/mm². Test results are shown in Table 3. As seenfrom the table, the rupture time of the base alloy was 34.8 hours.However, in the case where the base alloy was unidirectionallysolidified, the rupture time decreased to about a half, being 14.7hours. The reason for this that in the case of the unidirectionallysolidified alloy, there exist crystal grain boundaries which strength islow. In contrast, the Y-10C alloy containing additive Fe in an amount ofexhibited the rupture time of 32.1 hours, which was generally the sameas the rupture time of the single crystal base alloy.

TABLE 3 Rupture time elongation reduction of Alloy (h) (%) area (%) Basealloy/SC 34.8 25.6 30.4 Base alloy/DS 14.7 19.9 18.0 Y-10C/DS 32.1 17.825.1

Thus, it was revealed that the invention alloy containing Fe has aneffect of improving not only the oxidation resistance but also the grainboundary strength. From this, it can be appreciated that in thelarge-size rotor blade made of a single crystal, an allowable existencerange of hetero-crystals is expanded.

The invention alloy is suitable for products being directionallysolidified by the unidirectional solidification method. In particular,when producing rotor blades of gas turbines it is desirable to castthose with a solidification direction according with a direction ofcentrifugal force to be acted on the rotor blades. Although the abovedescription has been made on the basis of using the invention alloy inthe rotor blades of gas turbines, the invention alloy can be applied tostator blades of gas turbines and other type members used at a hightemperature. In the case where the invention alloy is applied to thestator blades, it is preferably cast with a solidification directionaccording with a direction of maximum thermal stress occurring in thestator blades. The invention alloy can be used for not only ordinarycolumnar rotor blades and columnar stator blades but also rotor bladesin which crystal grain boundaries occur partially in the rotor bladeduring casting as a single crystal. Such a rotor blade hasconventionally been regarded as a defective product. However, in thecase where the invention alloy is used, such a rotor blade can stand upto use. As a result, the casting yield of single crystal blades can beimproved significantly. Also, the invention alloy can be applied tousual single crystal rotor blades. Although there might be the casewhere complete single crystal rotor blades and complete single crystalstator blades, being made of conventional single crystal alloys, can becast with a high yield, according to the invention alloy, an inspectionwork for determining whether crystal grain boundaries exist can besimplified, so that the production cost can be saved. Further, althoughthe presence or absence of crystal grain boundaries in the inner surfaceof rotor blade has conventionally been ensured by the samplingdestructive inspection, according to the invention alloy, the strengthcan be ensured even if crystal grain boundaries exist, so that thereliability of the rotor blades can be improved significantly.

As will be apparent from the above, the invention Ni-based singlecrystal alloy can be used for members exposed to a high temperature,such as rotor blades and stator blades of high-temperature equipmentincluding a gas turbine.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A Ni-based single crystal alloy consisting essentially of, by weight,from not less than 0.06% to not more than 0.09% carbon, from not lessthan 0.016% to not more than 0.035% B, from not less than 0.2% to notmore than 0.4% Hf, from inclusive zero to not more than 0.02% Zr, fromnot less than 6.5% to not more than 8.5% Cr, from not less than 0.4% tonot more than 1.0% Mo, from not less than 5.5% to not more than 9.5% W,from not less than 1.2% to not more than 3.1% Re, from not less than 8%to not more than 10% Ta, from not less than 0.3% to not more than 1.0%Nb, from inclusive zero to not more than 0.4% Ti, from not less than4.7% to not more than 5.4% Al, from not less than 0.5% to not more than5.0% Co, from not less than 0.1% to not more than 5% Fe, and the balanceof Ni and unavoidable impurities.
 2. A Ni-based single crystal alloyaccording to claim 1, which consists essentially of, by weight, from notless than 0.06% to not more than 0.08% carbon, from not less than 0.2%to not more than 0.3% Hf, from not less than 6.9% to not more than 7.3%Cr, from not less than 0.7% to not more than 1.0% Mo, from not less than7.0% to not more than 9.0% W, from not less than 1.2% to not more than1.6% Re, from not less than 8.5% to not more than 9.5% Ta, from not lessthan 0.6% to not more than 1.0% Nb, from not less than 4.9% to not morethan 5.2% Al, from not less than 0.8% to not more than 1.2% Co, and thebalance of Ni and unavoidable impurities.
 3. A Ni-based single crystalalloy according to claim 1, which contains, by weight, from not lessthan 0.5% to not more than 3% Fe.
 4. A Ni-based single crystal alloyaccording to claim 1, which consists essentially of, by weight, from notless than 0.06% to not more than 0.08% carbon, from not less than 0.2%to not more than 0.3% Hf, from not less than 6.9% to not more than 7.3%Cr, from not less than 0.7% to not more than 1.0% Mo, from not less than7.0% to not more than 9.0% W, from not less than 1.2% to not more than1.6% Re, from not less than 8.5% to not more than 9.5% Ta, from not lessthan 0.6% to not more than 1.0% Nb, from not less than 4.9% to not morethan 5.2% Al, from not less than 0.8% to not more than 1.2% Co, from notless than 0.5% to not more than 3% Fe, and the balance of Ni andunavoidable impurities.
 5. A Ni-based single crystal alloy according toclaim 1, which contains, by weight, from not less than 1% to not morethan 3% Fe.
 6. A Ni-based single crystal alloy according to claim 1,which further contains, by weight, from 0.01% to not more than 0.2% Si.7. A Ni-based single crystal alloy according to claim 1, which furthercontains, by weight, from 0.05% to not more than 0.15% Si.
 8. A castingmade of a Ni-based alloy having the alloy composition as defined inclaim 1.